1. Field of the Invention
The present invention is related to a beam splitter for producing a plurality of light beams from a single beam.
2. Description of the Prior Art
A prior art beam splitter for producing a plurality of light beams is described in U.S. Pat. No. 4,125,865 and in Japanese Patent Laid Open Gazette No. 60-19101. FIG. 1 is a sectional view of a known beam splitter BS.sub.o. The beam splitter BS.sub.o comprises a transparent substrate 11, a total reflection film 12 and an anti-reflection film 17 which are formed on one surface of the substrate 11, and a plurality of semitransparent films 13-16 formed on the other surface of the substrate 11. The semi-transparent films 13-16 have different characteristics. A light beam L.sub.1 is introduced into the transparent substrate 11 through the anti-reflection film 17, and parts of the light beam L.sub.1 are emitted through the semi-transparent films 13-16 as outgoing beams L.sub.01, L.sub.02, .... Thus, the beam splitter BS.sub.o produces a plurality of light beams L.sub.01, L.sub.02, .. from a single light beam L.sub.1.
Desirably, the outgoinq beams L.sub.01, L.sub.02, ... should have equal intensities. In order to attain this, the transmission factors of the semi-transparent films 13-16 progressively increase. For example, as described in Japanese Patent Laid Open Gazette No. 60-19101, the semi-transparent films 13-16 are formed as multi-layers, with the number of layers progressively decreasing. Such films can be formed through deposition of TiO.sub.2 or SiO.sub.2. However, the thickness of each layer should be precisely controlled to attain desired transmission factors; such fabrication is time-consuming and costly.